In cPanel under mail you can specify what to do with mail that would normally be bounced. I have it set to forward to a certain account called "Spam" and I check it weekly to see if there are any that shouldn't be there.
Also, all domains, not just 3 letter domains, are subject to this. They could see your domain by going there and just use that and use relakjfldskf@yourdomain.com.
But yes, 3 letter domains have this more often than longer ones.
If your hosting company allows you to have a catch all email address generall *@yourdomain.com then this problem can be avoided at no extra charge. You can check the email box from time to time to determine whether some genuien emails dont land up der.
Also it has one more advantage, suppose somebody mistypes your email id ex webmaster@yourdomain.com mistyped as webmastr@yourdomain.com, you can still receive this email as the catch all email ID is active.